The use of supercritical fluids, such as supercritical carbon dioxide fluid, has been demonstrated to provide good results in the effort to replace undesirable volatile organic solvents presently used in many applications. For example, U.S. Pat. No. 4,923,720 describes a process and apparatus for coating substrates using a supercritical fluid as a coating diluent. However, the means to deliver a reliable and consistent supply of compressed liquified gas from conventional high pressure liquified gas cylinders to an application has been lacking to date. Prior to the system and method of the present invention, fluid recovery from cylinders is substantially and solely a function of the ambient temperature, with fluid recoveries decreasing with colder ambient conditions.
One method attempting to deliver liquified carbon dioxide from a cylinder to an application uses a siphon or "dip" tube to reach the bottom of an upright cylinder. Another method of attempting to consistently deliver liquified carbon dioxide from a cylinder comprises inverting the cylinder to place liquid in the cylinder at the cylinder outlet.
Unfortunately, both the siphon tube and the inversion method of withdrawing compressed liquified gas fail in practice when any of at least two conditions occurs. If the pressure drop in the flow line from the cylinder to the pump is greater than a few millibar, the liquid nature of the flowing fluid will cease, and a local vaporization will occur.
Also detrimental to deliver of compressed liquified gas is the formation of vapor due to temperature changes. If the temperature in the cylinder from which liquified gas fluid is being withdrawn falls below the ambient temperature, and therefore likely below the temperature of downstream delivery system components, vaporization of flowing fluid will probably occur. Unfortunately, any vaporization in the deliver system may cause delivery pumps to cavitate, thus causing the cessation of flow of fluid to the application.
Referring now to FIG. 1, the recovery of carbon dioxide from a conventional cylinder as a function of ambient (cylinder) temperature is depicted. It is seen that even at very high ambient conditions, less than about 70% of fluid is recovered.
From the above, it is clear that a reliable system and method for consistently supplying compressed liquified gas to an application using such fluid is much desired.